Data-acquisition systems and programmable-logic controllers
(PLCs) require versatile high-performance analog front ends that interface with
a variety of sensors to measure signals accurately and reliably. Depending on
the particular type of sensor and the magnitude of the voltage or current being
measured, the signal may need to be amplified or attenuated to match the
full-scale input range of the analog-to-digital converter (ADC) used for
further digital processing and feedback control.

Typical voltage measurement spans in data-acquisition
systems range from ±0.1
V to ±10
V. By choosing the correct voltage range, the user implicitly changes the
system gain to maximize the amplitude of the sampled voltage at the input of
the analog-to-digital converter (ADC), which, in turn, maximizes the
signal-to-noise ratio (SNR) and measurement accuracy. In typical
data-acquisition systems, signals that require attenuation and signals that
require amplification are processed by different signal paths. This usually
results in a more complex system design, requires extra components, and uses
more board space. Solutions that offer attenuation and amplification in the
same signal path generally use programmable-gain amplifiers and variable-gain
amplifiers, but these amplifiers do not usually offer the high dc precision and
temperature stability required by many industrial and instrumentation
applications.

One way to build a powerful analog front end that provides
both attenuation and amplification in a single signal path—and differential
outputs to drive high-performance analog-to-digital converters—is to cascade a
programmable-gain instrumentation amplifier (PGIA), such as the AD8250 (gain of 1, 2,
5, or 10), AD8251 (gain of 1, 2,
4, or 8), or AD8253 (gain of 1,
10, 100, or 1000), with a fully differential funnel (attenuating) amplifier,
such as the AD8475, in a circuit
similar to that shown in Figure 1. This solution offers simplicity,
flexibility, and high speed—along with excellent precision and temperature
stability.

The AD8475 high-speed, fully differential funnel amplifier
with integrated precision resistors provides precision attenuation of 0.4 or
0.8, common-mode level-shifting, single-ended-to-differential conversion, and
input overvoltage protection. This easy-to-use, fully integrated precision gain
block is designed to process signal levels up to ±10 V using a single +5 V
supply. As a result, it can match industrial-level signals with the
differential input voltage range of low-voltage high-performance 16-bit and
18-bit successive-approximation (SAR) ADCs with sampling rates of up to 4 MSPS.

The AD825x and AD8475, working together as shown in Figure
1, provide a flexible high-performance analog front end. Table 1 shows the gain
combinations that can be achieved, depending on the input and output voltage
range requirements.

Table 1. Input Voltage Ranges and Gains Possible with the AAD8475 in Combination with the AD8250, AD8251, and AD8253

DAQ Instrument Measurement Range (V)

Peak-to-Peak Voltage (V)

ADC Max Voltage per Input (V)

Overall System Gain

AD825x Gain

AD8475 Gain

Peak-to-Peak Voltage at ADC Input

AD825x Input Voltage Limit Needed
(to Protect ADC)

±10

20

4.096

0.4

1

0.4

8

10.24

AD8250 Gain

±5

10

4.096

0.8

2

0.4

8

5.12

±2

4

4.096

2

5

0.4

8

2.048

±1

2

4.096

4

10

0.4

8

1.024

±5

10

4.096

0.8

1

0.8

8

5.12

±2.5

5

4.096

1.6

2

0.8

8

2.56

±1

2

4.096

4

5

0.8

8

1.024

±0.5

1

4.096

8

10

0.8

8

0.512

±10

20

4.096

0.4

1

0.4

8

10.24

AD8251 Gain

±5

10

4.096

0.8

2

0.4

8

5.12

±2.5

5

4.096

1.6

4

0.4

8

2.56

±1

2

4.096

3.2

8

0.4

6.4

1.28

±5

10

4.096

0.8

1

0.8

8

5.12

±2.5

5

4.096

1.6

2

0.8

8

2.56

±1

2

4.096

3.2

4

0.8

6.4

1.28

±0.5

1

4.096

6.4

8

0.8

6.4

0.64

±10

20

4.096

0.4

1

0.4

8

10.24

AD8253 Gain

±1

2

4.096

4

10

0.4

8

1.024

±0.1

0.2

4.096

40

100

0.4

8

0.1024

±0.01

0.02

4.096

400

1000

0.4

8

0.01024

±5

10

4.096

0.8

1

0.8

8

5.12

±0.5

1

4.096

8

10

0.8

8

0.512

±0.05

0.1

4.096

80

100

0.8

8

0.0512

±0.005

0.01

4.096

800

1000

0.8

8

0.00512

Capabilities: Input Voltage Range and BandwidthThe maximum input voltage range for the AD825x family of
PGIAs is about ±13.5
V when operating on ±15-V
power supplies (the AD8250 and AD8251 provide additional overvoltage protection
of up to 13 V beyond the power-supply rails). In this application, the
effective limit on the PGIA input voltage range is set by the full-scale
voltage range of the ADC inputs and the signal path gain from the sensor to the
ADC. For example, the AD7986 18-bit, 2-MSPS
PulSAR ADC operates on a single 2.5-V supply, with a typical 4.096-V reference
voltage. Its differential inputs accept up to ±4.096 V (0 V to 4.096 V
and 4.096 V to 0 V on the inputs). If the overall gain of the analog front end
is set to 0.4, with the AD825x configured for a gain of 1 and the AD8475
configured for a gain of 0.4, the system can process an input signal with a
maximum magnitude of ±10.24
V.

To determine the combination of gain settings required in
any system, consider the full-scale input voltage of the ADC (VFS) and the
minimum/maximum current or voltage levels expected from the sensors.

The speed and bandwidth of this analog front end is
exceptional given its level of precision and functionality. The speed and
bandwidth capabilities of this circuit are determined by the following
combination of factors:

AD825x
slew rate: The AD825x slews at a rate between 20 V/µs and 30 V/µs depending on
the gain setting. The AD8475 slews at a rate of 50 V/µs, so the system is
limited by the AD825x slew rate.

Antialiasing
filter (AAF) cutoff frequency: This user-determined filter band-limits the
signal presented at the ADC inputs to prevent aliasing and improve the SNR of
the signal chain (see amplifier and ADC data sheets for details).

ADC
sample rate: The AD8475 can drive up to 4-MSPS converters with 18-bit
resolution.

Many data-acquisition and process-control systems measure
pressure, temperature, and other low-frequency input signals, so the dc
precision and temperature stability of the front-end amplifiers are critical to
the system performance. Many of these applications include multiple sensors
that are multiplexed to the amplifier inputs in a polling fashion. Typically,
the polling frequency is much greater than the bandwidth of the signal of
interest. When the multiplexer switches from one sensor to the next, the
voltage change seen by the amplifier inputs is unknown, so the design must
accommodate the worst-case scenario: a full-scale voltage step. The amplifier
must be able to settle from this full-scale step within the time allotted to
switching. This settling time also needs to be lower than the settling time
required by the ADC to sample and acquire the signal.

An antialiasing filter (AAF) is recommended between the
AD8475 and the ADC’s inputs. The AAF band-limits the signal and the noise
presented to the ADC inputs to prevent undesirable aliasing effects and to
improve the system SNR. Additionally, the AAF absorbs some of the ADC input
transient currents, so the filter also provides some isolation between the
amplifier and the ADC’s switched-capacitor inputs. Typically, the AAF is
implemented using a simple RC network as shown in Figure 1. The following
equations describe the filter bandwidth:

In many cases, the filter’s R and C values are optimized
empirically to provide the necessary bandwidth, settling time, and drive
capability for the ADC. Refer to the ADC data sheet for specific
recommendations.

ConclusionTogether, the AD8475 and AD825x family of PGIAs implement a
simple analog front end that provides high performance, functionality, and
flexibility. A variety of programmable gain combinations are possible for both
amplification and attenuation, allowing different measurement voltage ranges to
be optimized. The AD825x’s performance and programmability are well-suited for
multiplexed measurement systems, and the AD8475 provides an excellent interface
to precision analog-to-digital converters. The two amplifiers work well
together to retain the integrity of the sensor signal, as a high-performance
analog front end for industrial measurement systems.

Reem Malik [reem.malik@analog.com]
is an applications engineer in the Integrated Amplifier Products (IAP) Group in
Wilmington, MA. She supports customers in the instrumentation, industrial, and
medical areas and is responsible for thermocouple amplifiers and precision
difference and differential amplifiers. Reem holds BSEE and MSEE degrees from
Worcester Polytechnic Institute. She joined Analog Devices in June 2008.(return to top)

Sandro Herrera [sandro.herrera@analog.com]
is a circuit design engineer in the Integrated Amplifier Products (IAP) Group
in Wilmington, MA. His design work currently focuses on fully differential
amplifiers with either fixed, variable, or programmable gains. Sandro holds
BSEE and MSEE degrees from the Massachusetts Institute of Technology. He joined
Analog Devices in August 2005.(return to top)

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